In a CMP process, a wafer with a weaker structure in mechanical strength, such as a single damascene via structure with a pattern density less than 10%, may suffer serious edge peeling or damage due to the wafer edge being continuously hit against the inside of a retainer ring of a carrier head during the CMP process. For example, weak interfaces, such as with an extremely low-k dielectric material with a dielectric constant k in the range of 1.5-2.5, may delaminate during, e.g., a CMP process, and result in a serious defect.